The present invention relates to switches, and more particularly to switches of the type that produce a signal, i.e., closure of contacts, upon detection of any one or more of the conditions of sudden movement, angular tilt, or vibration.
A substantial number of switches capable of sensing one or more of these conditions are presently available on the market. One technique for sensing movement or angular tilt that is commonly employed in switches of this type utilizes a conductive ball or roller disposed on an inclined or curved surface. In a normal position, the ball or roller is spaced from one of the contacts of the switch due to the design of the surface on which it rests. However, when the switch is moved or tilted, the ball rolls along the inclined or curved surface to a point where it electrically bridges the gap between two contacts to close them. In a variation of this technique, a pool of mercury is substituted for the ball or the roller.
In another type of movement sensitive switch, a movable contact, such as a metal rod, is pivotally suspended such that its free end, remote from the pivot point, is disposed proximate, but spaced from, a stationary contact. For example, the stationary contact can be a metal ring that surrounds the free end of the rod. In a switch of this type, the switch contacts are closed when the structure to which the switch is attached is suddenly jerked, wherein the inertia of the movable contact brings it into contact with the fixed contact, or when the switch is tilted to cause the pivot point to rotate relative to the fixed contact, thereby bringing the two contacts into engagement with one another.
For the most part, prior art switches for detecting movement, angular tilt or vibration have been designed with a specific application in mind. Therefore, the switch has a built-in sensitivity to a particular condition, e.g., angular tilt, that is required for the desired application. Attempts to utilize the switch in an application different from that for which it was designed are not usually successful, because the switch is either too sensitive or not sensitive enough for the new application. Adjustment of the sensitivity of the switch is often difficult, if not impossible. For example, in switches of the type utilizing a ball on an inclined or curved surface, the angle of inclination or radius of curvature of the surface, and perhaps the size of the ball or roller, would have to be changed in order to alter the sensitivity of the switch. Likewise, in a switch of the type utilizing a suspended rod surrounded by a stationary metal ring, the radius of curvature of the ring, or the distance between the pivot point and the ring must be changed. Each of these types of changes require substantial alteration to the structure of the switch itself.
One particular field in which it is desirable to provide a motion sensitive switch having easily variable sensitivity is in connection with anti-theft and anti-intrusion alarm devices. For example, if the alarm device is intended to be used on typewriters, the amount of vibration that is imparted to the device during normal typewriter operation will vary in dependence upon the particular type of typewriter and the type of support surface on which it rests. The ability to adjust the sensitivity of the switch provides a significant advantage, since the sensitivity of the alarm device can be set at a level which will enable it to operate effectively at a particular location without being triggered by normal everyday operation.
It is therefore a general object of the present invention to provide a novel switch that is capable of providing an electrical signal in response to detection of angular tilt, movement and vibration.
It is another object of the present invention to provide a novel switch of this type in which the sensitivity of the switch to the conditions to be detected can be easily adjusted without requiring a change in switch structure.
It is a further object of the present invention to provide a novel switch having general applicability to a variety of different types of anti-theft and anti-intrusion alarm devices.
It is yet another object of the present invention to provide a novel motion-sensitive alarm system.
A switch that is responsive to angular tilt, having adjustable sensitivity to the degree of angular tilt required to close the switch contacts, is disclosed in U.S. Pat. No. 3,786,469. The switch disclosed in this patent has a movable contact comprised of a bob that is suspended by a chain within a stationary contact comprised of a conically-shaped cup. The end of the chain remote from the bob is attached to a pivotable support structure which enables the vertical position of the bob within the cup to be adjusted to thereby vary the sensitivity of the switch to angular tilt. The specific purpose for the switch that is disclosed in the patent is to detect wave motion, and therefore the switch is responsive to only relatively large angular variations.
Although the sensitivity of the switch can be adjusted, it is not generally suited for application in a variety of anti-theft devices, due in large part to its relative insensitivity to vibration. This insensitivity is a result of the dampening effect that the chain and the weight of the bob have on vibrations. In other words, only vibrations of a significant magnitude sufficient to tilt the structure on which the switch is mounted would cause the suspended bob to come into contact with the stationary cup. Smaller vibrations would be damped out by the chain and not transmitted to the bob. Furthermore, the inherent electrical resistance of a bead chain does not render it suitable for use as an electrical conductor, and it can result in faulty operation of the circuit in which the switch is incorporated. Therefore, a switch of this type would not be suitable for use in an anti-theft alarm device that is attached to typewriters, for example, wherein it is possible for a thief to vertically lift a typewriter and slowly carry it away without tilting the typewriter or jerking it by an amount sufficient to close the contacts of the switch. However, it is often quite difficult to carry a typewriter away without imparting at least some vibratory movement to it.
It is therefore yet another object of the present invention to provide a novel switch that is responsive to both angular tilt and vibration to close its contacts upon the occurrence of either one of these conditions, to thereby render the switch suitable for use in a wide variety of anti-theft alarm applications.